We would predict, for example, that bystander effects for NTR-armed Clostridia sporogenes (Lemmon et al, 1997) localised in necrotic regions of tumours will depend to a greater extent on the 2-amine metabolite (3), while the 4-hydroxylamine (4) will make a larger contribution in tumours grown as intimate mixtures of NTR+ve and −ve cell lines. This evidence concerns the gene NTSR1 and neoplasm.